qmp: Deprecate 'singlestep' member of StatusInfo

The 'singlestep' member of StatusInfo has never done what the QMP
documentation claims it does.  What it actually reports is whether
TCG is working in "one guest instruction per translation block" mode.

We no longer need this field for the HMP 'info status' command, as
we've moved that information to 'info jit'.  It seems unlikely that
anybody is monitoring the state of this obscure TCG setting via QMP,
especially since QMP provides no means for changing the setting.  So
simply deprecate the field, without providing any replacement.

Until we do eventually delete the member, correct the misstatements
in the QAPI documentation about it.

If we do find that there are users for this, then the most likely way
we would provide replacement access to the information would be to
put the accelerator QOM object at a well-known path such as
/machine/accel, which could then be used with the existing qom-set
and qom-get commands.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 20230417164041.684562-11-peter.maydell@linaro.org

docs/about/deprecated.rst

@@ -199,6 +199,20 @@ accepted incorrect commands will return an error. Users should make sure that
 all arguments passed to ``device_add`` are consistent with the documented
 property types.
 
+``StatusInfo`` member ``singlestep`` (since 8.1)
+''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``singlestep`` member of the ``StatusInfo`` returned from the
+``query-status`` command is deprecated. This member has a confusing
+name and it never did what the documentation claimed or what its name
+suggests. We do not believe that anybody is actually using the
+information provided in this member.
+
+The information it reports is whether the TCG JIT is in "one
+instruction per translated block" mode (which can be set on the
+command line or via the HMP, but not via QMP). The information remains
+available via the HMP 'info jit' command.
+
 Human Monitor Protocol (HMP) commands
 -------------------------------------
 

qapi/run-state.json

@@ -104,16 +104,24 @@
 #
 # @running: true if all VCPUs are runnable, false if not runnable
 #
-# @singlestep: true if VCPUs are in single-step mode
+# @singlestep: true if using TCG with one guest instruction
+#              per translation block
 #
 # @status: the virtual machine @RunState
 #
+# Features:
+# @deprecated: Member 'singlestep' is deprecated (with no replacement).
+#
 # Since: 0.14
 #
-# Notes: @singlestep is enabled through the GDB stub
+# Notes: @singlestep is enabled on the command line with
+#        '-accel tcg,one-insn-per-tb=on', or with the HMP
+#        'one-insn-per-tb' command.
 ##
 { 'struct': 'StatusInfo',
-  'data': {'running': 'bool', 'singlestep': 'bool', 'status': 'RunState'} }
+  'data': {'running': 'bool',
+           'singlestep': { 'type': 'bool', 'features': [ 'deprecated' ]},
+           'status': 'RunState'} }
 
 ##
 # @query-status: